Various methods and apparatus for focusing waves are known. Basic principles of wave physics suggest a fundamental minimal limit for the dimension of a high amplitude region or "spot" to which a wave is focused. In practice, it is difficult to approach this fundamental minimal limit due to the physical characteristics of various elements used to focus a wave. A well-known limit to wave focusing arising from the finite dimensions of a focusing element is referred to as the diffraction limit.
Super-resolution techniques for achieving a reduced focus spot size smaller than a diffraction limited spot size involve placing slit-like or annular apertures in a wave path. The wave is diffracted from two apertures formed in a light intercepting plane which present an intensity distribution in a plane beyond the apertures. Such an intensity distribution is a well-known diffraction interference pattern. The diffraction pattern comprises a main high intensity lobe flanked on either side by a series of "sidelobes" having a decreasing intensity as a function of the distance from the main lobe. The spot size of the main lobe of the diffraction pattern is smaller than the diffraction limited spot size of a wave passing through a single aperture. However, the presence of the sidelobes is undesirable for many applications, because they limit the resolving ability of the main lobe spot.
Other super-resolution approaches to wave focusing employ various filtering or masking techniques near the face of an interference or "focal" plane to reduce the presence of sidelobes to a certain extent. However, such techniques typically present a problem in that, while wave energy in the sidelobes is decreased, the amount of wave energy generally reaching the focal plane is also markedly decreased in the central main lobe. Other techniques for suppression of sidelobes involves phase shifting devices, typically in the form of phase plates or zone plates, through which a wave is passed prior to being focused by a focusing element. Such plates induce phase reversals in select spatial portions of a wave, such that the sidelobes of the interference pattern in a focal plane are canceled.
For several optical applications, yet other approaches to reducing a focused wave spot size have been proposed, some of which involve beam obscuration devices. An example of one such approach is hyper-resolution focusing, wherein light is blocked from passing through a central portion of a focusing lens. In this technique, only the light beam passing through the peripheral portion of the lens is focused. In some sense, the peripheral portion acts like an aperture, as described above, and the blocked lens generates a diffraction pattern in which secondary peaks appear on both sides of a primary peak in the focal plane. Similar to the interference pattern from the apertures described above, the primary peak is narrower than when the light is not blocked from passing through the central portion of the lens, but secondary diffraction peaks, or sidelobes, remain in the focal plane.
Accordingly, while known approaches reduce the focus spot size beyond the diffraction limit, they may not provide adequate sidelobe suppression and may additionally require other combinations of techniques for suppression of sidelobes, as discussed above. However, many schemes for reducing sidelobes also reduce wave energy in the central main lobe. Additionally, solutions to wave focusing using complex schemes of focusing elements often reduce a depth of focus of the focusing system. Depth of focus refers to distance along the direction of wave propagation over which a beam remains focused. For many applications, lens systems having a low depth of focus are a disadvantage, requiring greater focusing precision to properly direct a beam to the medium of a sample.
It is therefore advantageous for a focusing system to be able to focus a wave to a small central spot having a minimum dimension less than the diffraction limited spot size, and more preferably approaching a fundamental minimal limit, without the presence of secondary spots or "sidelobes" which adversely affect the resolution of the central spot. It is further advantageous for such a focusing system to be able to achieve different depths of focus, thereby finding use in various applications benefitting from both high and low depths of focus.